Three stage electron lens system excited by permanent magnets



March 8; 1960 BUNYA TADANO ETAL 2,928,004

THREE STAGE ELECTRON LENS SYSTEM EXCITED BY PERMANENT MAGNETS 2 Sheets-Sheet 1 Filed June 22, 1955 Fl9.l

MAGNETIC MATERIAL 2 m7 1 w B M -jMfiCMZR n o D 0 I March 8, 1960 BUNYA TADANO ETAL 2,923,004

- THREE STAGE ELECTRON LENS SYSTEM EXCITED BY PERMANENT MAGNETS 2 sheets sheet 2 Filed June 22, 1955 C e Mm n 5 I 8 MW" a m l m m e d i IC m I. rLe T e d e7. e .1. Pb n w F I vr United States Patent STAGE ELECTRON LENS SYSTEM EX- CITED BY PERMANENT MAGNETS Bunya Tadano, Nozomu Morito, and Hiroichi Kimura, Tokyo, Japan, assignors to Hitachi Limited, Tokyo, Japan Application June 22, 1955, Serial No. 517,186

Claims priority, application Japan August 27, 1954 1 Claim. (Cl. 250-495) This invention relates to magnet electron lens systems to be used for electron microscopes and similar devices.

It is an object of the invention to provide an improved system which facilitates focusing, has a variable magnification over a wide range, and can obtain a diffraction pattern from a limited area.

To achieve the above and other of its objectives, the invention provides a three-stage electron lens system having objective, projection and intermediate lenses. The intermediate lens according to a feature of the invention is excited by the difference in magnetomotive forces in the other lenses which are arranged in magnetically bucking relation.

For a better understanding of the invention, the details of a preferred embodiment will next be described with reference being made to the accompanying drawings, in which:

Fig. 1 is a sectional view of a magnetic electron lens embodying this invention;

Fig. 2 is a fragmentary transverse section taken through the line A-A of Fig. 1;

Fig. 3 shows explanatory curves;

Figs. 4a, b, c and d show diiferent electron paths in the lens system shownin Fig. 1; and

Fig. 5a and b are plan views of a set of rings for regulating leakage flux.

In Fig. l, 1 designates on objective lens which is excited by the cylindrical permanent magnet 4, and 3 designates a projection lens excited by the cylindrical permanent magnet 5. The magnetic circuit is established, on one hand, through the objective lens 1, upper disc 16, external shield cylinder 12, central disc 17, permanent magnet 4 and the yoke 20 attached to magnet 4, lens 3 under disc 19, the external shield cylinder 13, central disc 18, permanent magnet 5 and the yoke 21 attached to magnet 5. Two magnetic circuits are arranged in bucking relation and the same poles of permanent magnets 4 and 5 are joined through the central discs 17 and 18. An intermediate lens 2 is located at the middle of the inner shield yokes 14 and 15. The elements are supported in position within cylinders 12 and 13 by means of a cylindrical non-magnetic spacer 30 constituted by said yokes.

The permanent magnets 4 and 5 have substantially equal magneto-motive forces, and almost no magnetomotive force is imposed on intermediate lens 2 so that the intermediate lens does not generally efiect a lens action. Thus, a two stage magnification of an image can be effected by the objective lens 1 and the projection lens 3. In this case, the electron paths are as shown in Fig. 4a.

Fig. 3 shows curves illustrating the relations between the magneto-motive force of the projection lens and magnification Mp of the projection lens, the resultant magnification Moi by the objective lens and intermediate lens and also the magnification M of final image as an ice example, in which the working point is at the point a to give the maximum magnification.

Referring to Fig. 1, on the side of projector lens 3 there is provided a set of concentric rings 9 and 10, of which the ring 10 is fixed, whereas the ring 9 can be turned by means of a gear wheel 11. There is a cylindrical gap between the rings 9 and 10. These rings 9 and 1d are made of nonmagnetic material having ferromagnetic pieces 26 and 27 embedded therein as shown in Fig. So. When the cooperating sets of ferromagnetic pieces are brought to coincident positions, there occur maximum stray fluxes so that the magneto-motive force acting on the projection lens becomes minimum and the focal length of the projection lens is longest. On the other hand, there results a difference between the magneto-motive force of the permanent magnet on the side of the objective lens and that on the side of projection lens, which acts on the intermediate lens, and the working point in Fig. 3 is shifted to the left so that the magnification is decreased. In this case, the electron paths will become as shown in Fig. 4b.

As the difference between magneto-motive forces becomes larger, the working point will be shifted to b in Fig. 3, and the back focus of the objective lens will be located in front of the projection lens. By operation of the field liming aperture provided between the intermediate and objective lenses, it is possible to obtain a final image of dilfraction pattern from a limited area of the specimen in the objective stage of the electron microscope. The electron paths are as shown in Fig. 40. In this case, the effect of variation in the magneto-motive force of the projection lens upon the magneto-motive force of the objective lens can easily be made less than 2 to 3% over the whole range 'of variation, because of the extremely large permeance at the ends of the permanent magnets and large total fluxes for exciting the objective lens compared with the variation of the fluxes passing through the intermediate lens. Moreover, the demagnetizing field acting between the permanent magnets would never actually be the cause of demagnetization which can be avoided by the proper selection of magnetic materials and suitable design of magnetic circuits. With further respect to the design, it is preferable to make the permeance of the intermediate lens small.

The foregoing description is of an electron lens system excited by permanent magnets as shown in Fig. 1, in which the diiference between electro-motive forces of two sets of permanent magnets is substantially zero. It is also possible to use a lens system having two sets of permanent magnets with a difference in magneto-motive force. In such a case, an image of higher magnification can be obtained since the intermediate lens can be used for magnification. The electron paths are as shown in Fig. 4a.

For focusing the objective lens, a pair of rings 7 and 8 as shown in Fig. 1 may be used, of which the ring 8 is fixed, whereas the ring 7 is rotatable by means of a gear wheel 6. There is a gap between the rings 7 and 8. These rings are made of non-magnetic material ring '7 (or 8) having ferromagnetic pieces 28 embedded therein as shown in Fig. 2, and when the ferromagnetic pieces of these rings are made to coincide with each other by turning the ring relative to the other, the stray fluxes will become maximum so that the working point on the demagnetization curve of the permanent magnet is shifted on the minor hysteresis loop in the direction in which the coefficient of permeance becomes larger. Accordingly, the magneto-motive force decreases and the focal length of the objective lens becomes longer. If, on the other hand, the ferromagnetic pieces are in staggered relation, the stray fluxes will become minimum and the focal length of the objective lens will be shortened. In either of the above cases, the shifting of working point on the minor hysteresis loop seems to be substantially reversible for certain materials so that the magnet is never demagnetized.

In general, it is desirable to provide rough and fine adjusting apparatuses.

What we claim is:

An electron lens system comprising two electron lens units located in magnetically bucking relation and constituting respectively an objective lens and a projection lens, said lens units each comprising two discs, an outside shield cylinder housing said discs and a permanent magnet with poles between said discs and exciting the associated lens through said tWo discs and outside shield cylinder, and an intermediate lens between said objective lens and projection lens and including one pole piece References Cited in the file of this patent UNTTED STATES PATENTS 2,503,173 Reisner Apr. 4,

2,579,273 Reisner Dec. 18, 1951 2,714,678 Wolff Aug. 2, 1955 2,761,991 Ersfeldt Sept. 4, 1956 2,858,443 Kimura et a1. Oct. 28, 1958 s. My. 

